Post-tensioned concrete is a variant of pre-stressed concrete where the tendons or cables are tensioned after the surrounding concrete structure has been cast. Construction of post-tensioned slabs on grade is very similar to using reinforcing steel, except for the tensioning step. Cables are arranged as indicated by the engineer and typically chaired to run through the center of the slab. For residential construction, tendons at 48 inches on center are common. Commercial foundations will have much more steel. Tendons can be easily routed around obstructions.
A residential post-tensioned concrete slab will typically be 8 inches thick and use 3000 psi concrete. Once the concrete has gained strength to 2000 psi, typically within the 3 to 10 days recommended by PTI, the tendons are stressed.
Tendons today are seven high-strength steel wires wound together and placed inside a plastic duct. At each end of the tendons, a post tension anchor is positioned for securing the cable or tendon and to distribute tensile force into the concrete by applying tension to the tendons. One end of the cable tendons requires a pocket to be formed so that stressing equipment can access the anchor for force application and wedge seating operations. When the strands are stressed, the tendons will stretch—about 4 inches for a 50 foot strand—to apply 33,000 pounds of load. Stressing of post-tension tendons is a dangerous job that requires a lot of skill and is typically only done by qualified workers. After stressing, the tendon is cut off and a grease cap is installed in encapsulated systems to create a watertight seal and then the pockets are filled with non-shrink grout to protect them from corrosion.
Larger structural concrete members may also be post-tensioned, especially in bridges and floors and beams in parking structures. The process is very similar to that used for slabs, except on a bigger scale. One interesting difference is that the tendons will often be “draped” so that they are low at the midpoint of a beam and high at the support, which places the steel at the point of highest tension where it can keep the concrete held together tightly. In multi-strand applications, with structural members the duct is often grouted full following stressing to bond the strand to the concrete along its entire length. When the duct is fully grouted, the combined duct, grout and tendons are called bonded tendons. On the other hand, in mono-strand applications, the duct is normally not grouted. Unbonded tendons, like those used in residential slabs and commercial low rise buildings, remain free to move within the duct and are protected from corrosion by grease.
Post-tension tendon placement and stressing is usually done by companies with certified workers who specialize in this work. Current pocket formers for post tension cables only provide a small alignment ring for the anchor to plywood connection. This requires the user to hold the anchor in alignment and secure it using nails or screws. Holding the anchor, nail and a hammer is difficult in open situations but even harder in tight spaces, such as beams and columns with rebar. Others have used a threaded mandrel and nut that secures the anchor to the plywood formwork, but this still requires two people in areas where the opposing side of the formwork is not easily accessible. The mandrel approach also requires multiple pieces which makes it cumbersome.
As such, currently the pocket former and anchor assembly require nails or screws to hold the assembly in place during tendon installation and concrete placement. The installation of nails into the plywood is difficult because of rigidity, and having to hold everything at one time. In addition, the use of nails only typically results in some back-outs of the assembly (ring shank nails may be used to limit this possibility), which takes constant supervision and time to deal with. Screws make this easier, but can make removal of the forms much more difficult, as the form has to be ripped off the head of the screws due to the head of the screws being buried in the concrete (i.e. the screws can not be unscrewed). As a result, both screws and nails are difficult to install and require extensions and creative methods when rebar is installed before the tendons are installed. Therefore, there is clearly a need to provide a means and method for faster and easier installation of the pocket former and anchor assembly with post-tension cables.
The instant disclosure is designed to address at least certain aspects of the problems or needs discussed above by providing a tendon pocket former with a ribbed formwork securing mechanism and a method of use thereof.